Smoke Flow Type "Fire" vs "Fire+Smoke"
Flow type 

"Fire" -  lets the flame generate first and then the smoke is created when the flame dies off (depending on your settings). The smoke is generated FROM the FIRE not the mesh. 

"Fire+Smoke"- Generates fire AND smoke at the same time. Does not wait for the flame to die. The MESH generates the fire and smoke at the same time.